One of the basic steps in the fabrication of a wide variety of semiconductor microelectronic devices involves doping of silicon with an impurity, commonly referred to as a dopant, to produce in the silicon a region, such as a buried layer or channel, of desired semiconductor type, i.e. n-type or p-type. An n-type silicon is one in which the majority charge carriers are electrons and is produced by doping with an n-type silicon dopant of the donor species selected from Group V of the Periodic Table of Elements. A p-type silicon is one in which the majority carriers are holes and is produced by doping with a p-type dopant of the acceptor species selected from Group III of the Periodic Table of Elements. By way of example, one type of semiconductor microelectronic device whose fabrication involves doping of silicon is an NPN transistor having a doped region of low resistivity silicon in a silicon substrate buried under a layer of epitaxial silicon. This buried region is called a buried layer and is produced by diffusing the dopant into the silicon substrate to form in the substrate a region, referred to herein as a buried region, which is diffused with the dopant and constitutes the buried layer. Other buried dopant diffused regions are called buried channels. In the context of this invention the expression "buried region" is intended to cover, in a generic sense, all diffused dopant regions in a silicon substrate produced by diffusing a dopant into the substrate.
At the present time, antimony is used as the dopant for the buried layers of NPN transistors. The reasons for selecting antimony as the dopant are twofold; namely, the low diffusion coefficient of antimony in silicon and the capability of diffusing antimony by a gaseous process to a relatively high (0.005.OMEGA.-CM) dopant level. The use of antimony as a dopant for buried layers, however, does present certain problems. For example, antimony for use as a dopant is derived from SbO.sub.3 by decomposing the latter in a source furnace at .about.900.degree. C. This source furnace requires substantial maintenance, is subject to failure, and necessitates the use of a special diffusion tube. Moreover, antimony diffusion requires a thick (.about.18,000 A) oxide layer to mask the diffusion. Growing this thick oxide layer creates more crystal defects than a thin oxide layer. The thick oxide layer also degrades photoresist etching resolution due to the long etch times required to remove the thick oxide. Another problem resides in the fact that the antimony diffusion junction depth and sheet resistance have poor repeatability run to run. Finally, gaseous antimony diffusion requires an oxide mask on the back of the substrate, thus complicating buried layer photoresist procedures.
Arsenic also has a high solubility limit and a low diffusion coefficient in silicon and does not present any of the antimony diffusion problems discussed above. Based on the foregoing factors, therefore, arsenic is an excellent buried layer dopant for silicon. Unfortunately, however, the vapor pressure of arsenic into silicon is too low to achieve a satisfactory dopant level on the order of 0.005.OMEGA.CM using an open tube diffusion procedure. While such a dopant level can be attained by use of a closed tube diffusion procedure, the existing closed tube procedures are very complicated and hence costly and time consuming to practice. It is also possible to employ a spin-on arsenic dopant source in an open tube diffusion procedure. Such spin-on sources, however, require special handling, involve operator exposure to arsenic, are sensitive to humidity, have limited shelf life even when stored under refrigeration, and produce non-uniform diffusions in oxide mask windows. Silicon doping has also been accomplished by providing a doped layer of polycrystalline silicon on a silicon substrate and heating the layer and substrate to diffuse the dopant into the substrate. In this procedure, no attempt was made to oxidize the polysilicon from the substrate during diffusion. In this regard, it is significant to note that total removal of the polysilicon is necessary if an epitaxial layer is to be grown over the buried region.